The embodiments described herein relate generally to rotor assemblies and, more particularly, to a rotor assembly that is biased to rotate in one direction.
At least some known pumps are synchronous pumps that include a permanent magnet therein. Such pumps typically do not have a preset direction of spin, and can rotate either clockwise or counter-clockwise when the pump is started. More specifically, alternating current (AC) is used to power the pump, and a rotor assembly includes a substantially cylindrical magnet having a first half with a north polarity and a second half with a south polarity. Since the power driving the pump is based on alternating current, the magnetic field supplied by the stator assembly is constantly changing polarity. When the AC power is applied to the stator winding, the stator assembly develops a magnetic field. If the stator winding's poles align with the rotor magnet's poles, the rotor assembly will rotate as the “like” paired poles push against each other, or repel each other. If the stator winding's poles are out of phase with the rotor magnet's poles, the rotor assembly will rotate to a state where the oppositely paired poles align, or attract each other. Further, if poles of the rotor assembly align adjacent to an opposite pole of the stator assembly, the rotor assembly may not begin rotating because the poles attract each other. Such a position of rotor assembly is referred to as a null position.
Because the first alignment of the poles of the rotor magnet and stator winding is random, the direction of impeller rotation is also random. Inertia of the rotor assembly maintains rotation of the impeller in one direction once the rotor assembly begins rotating. Such synchronous pumps are relatively inexpensive. However, because of the equal probability of spin direction, impeller efficiency must be sacrificed to provide equal flow rates in either spin direction. More specifically, such pumps usually include impellers having straight blades that are equally efficient in either spin direction.
Known induction pumps are more expensive than permanent magnet synchronous pumps, but have higher efficiency than synchronous pumps. More specifically, at least some known induction pumps only allow rotation of an impeller in one direction. As such, induction pumps include contoured or curved blades that are more efficient in one rotation direction than the other rotation direction. However, such contoured or curved blades cannot be used with known synchronous pumps because of the random rotation direction of the impeller. Accordingly, permanent magnet synchronous pumps can not typically match the performance of an induction pump, given the same power rating.